Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

Enterprising epidemiologist David Van Sickle at the University of Wisconsin-Madison decided to find out. He recruited four asthmatic undergraduates to carry around inhalers outfitted to relay location data when they were being used, via the Global Positioning System satellite network.

The test successfully demonstrated the concept and gave Van Sickle footing — and funding — for a pilot program now under way in the city of Madison, Wis. So far 19 volunteers have signed up to participate, with slots for another 31 available.

Advances in GPS technology solved what once would have been the program's main technical hurdle — hefty receivers. Tracking devices have become so small they can be attached onto a bird's leg.

"At one time, I was worried that lugging this inhaler around would cause people to have asthma attacks. It looked like a washing machine tied on to an inhaler," Van Sickle told Discovery News.

The device is now about the size of a nine-volt battery. And the weight, said Van Sickle, "is insignificant."

Current versions have additional technology to relay positioning data from inside buildings, a constraint that impacts most satellite-only receivers.

Eventually, Van Sickle, who previously worked for the Centers for Disease Control in Atlanta, would like to have thousands of people using the GPS-equipped inhalers so that accurate statistics can be complied about when and where asthma strikes.

"Asthma is unique in that people carry their inhalers around with them and use them at the time and place when they are having symptoms," Van Sickler said.

While at the CDC, Van Sickle tried to track asthma outbreaks by getting hospital emergency room statistics about asthma treatments, but the key piece of information — figuring out where the attacks began — remained a mystery.

The goal of the Madison project is to learn something about asthma exposures, but it also is helping individuals better understand what triggers their symptoms.

"We had this one guy who was using his inhaler every day at work, and he was fine the rest of the time. He had never put it together that he had workplace-related asthma. It's funny what people miss when they're so close to stuff," Van Sickle said.

GPS-endowed medical devices are a new but growing part of GPS applications, according to Oregon-based Glen Gibbons, who has been tracking the industry since 1989.

"There's a system being developed to track Alzheimer patients and one that can track when someone takes a fall," he said.